From Alaska to the Andes, glaciers all over the world have been retreating for decades, as average global surface temperatures have increased. This loss of these glaciers is one of the most iconic manifestations of manmade global warming, but until now, no one had studied the obvious question: Just how much global glacier melt (referred to technically as “glacier mass loss”) is global warming responsible for, and how much is from natural climate variability?

A new study, published Thursday in the journal Science Express, tackles that question, and comes to a profound — if not surprising— conclusion. The study found that manmade global warming, which is largely due to the burning of fossil fuels such as oil and coal for energy, is responsible for nearly 70% of global glacier mass loss between 1991 to 2010.

“Glaciers are melting because of human actions, which implies that also related impacts, such as changed water availability and hazards from glacial lakes can be considered man-made,” lead author Ben Marzeion, with the University of Innsbruck, told Mashable.

The study, by researchers at the University of Innsbruck and Trent University, found that over a longer time period, from 1851-2010, the manmade signal is smaller, accounting for about 25% of glacier melt. Part of the reason for this is that during the early decades within that time, period glaciers were still responding to natural climate variability, including the end of the so-called “Little Ice Age” when temperatures were considerably cooler than they are now.

Meltwater channels from the previous summer and terminus of the Violin Glacier in East Greenland, seen during an Operation IceBridge survey flight on April 5, 2014. Image: NASA IceBridge

The study showed that glaciers are still responding to climate change that occurred years ago because they have a delayed response to warming. This means that the accelerated warming seen since the 1970s is likely to cause even more melting in the coming years, even though temperatures rose much more slowly during the past decade.

Melting glaciers are a hazard because they contribute to rising sea levels, which are already causing coastal storms to be more destructive, and also altering water resources; this leads to flooding hazards in many areas, such as the Himalayas. Without manmade global warming, glaciers would have contributed about 3.9 inches to global sea level rise during the 1851 to 2010 period, the study found; but with it, they contributed 5.2 inches.

This may sound like a small number, but every fraction of an inch can make a huge difference when it comes to storm surge in highly populated coastal cities. For example, the foot of sea level rise during the past century in New York City was enough for Hurricane Sandy’s storm surge to flood thousands of additional homes and businesses than the storm would have without that extra water.

The Anderson Glacier in Olympic National Park in 1936, and again in 2004. Credit: National Park Service. Image: National Park Service

The study said that in some areas, it is quite clear that manmade global warming is driving glaciers to shrink, including Alaska, western Canada, Arctic Canada, Greenland and north Asia, among others. But in some spots, such as the southern Andes and Caucasus region, the study’s methods did not detect a manmade warming signal with high confidence.

To arrive at their conclusions, the researchers used a model of global glacier evolution based in part on the Randolph Glacier Inventory, which contains information for individual glaciers, as well as the latest global climate models to simulate the contribution of natural and manmade climate change. They found that the computer model simulations could not match the observed record without including manmade global warming. The study includes all of the world’s glaciers outside of Antarctica.

Richard B. Alley, a geosciences professor at Pennsylvania State University who was not involved in the new study, described glaciers as “slow thermometers.” In an email to Mashable, he said:

Stick a fever thermometer in your mouth, and it takes a little while to adjust to the new environment and give the accurate temperature. In the same way, if there were a temperature change in a single step, with the temperature then held constant at that new level, a glacier will take a while to come into balance with the new temperature…

High scientific confidence that the human-dominated part of the warming is primarily responsible for glacier retreat thus doesn’t emerge until late in the 20th century, even though the “most likely” answer is that humans have been contributing at least a little for over a century, with a growing influence more recently.

Alley added that the study’s results “make perfect sense.”

“Warming melts glaciers, whether the warming is caused by natural or human causes. And because glaciers are slow thermometers, even if humans were to quit warming the climate, the glaciers will lose more mass in the future as they “catch up” with the warming that has already occurred,” he said.

Eric Steig, a University of Washington professor who was not involved in the new study, said it is “convincing, but not at all surprising.” He added that the lagged response of glaciers doesn’t go back more than a few decades for most small glaciers, and about a century for larger ones.

Marzeion, the study’s lead author, cautioned that there are still uncertainties about how some glaciers are responding to climate change, and improvements also need to be made to computer models of the climate. Those caveats, however, do not detract much from the bottom-line message that manmade global warming is now the primary reason why glaciers are melting.

SINGAPORE, Jan 31 (Reuters) – The breaking off of a Luxembourg-sized iceberg in Antarctica could affect ocean circulation patterns and be a harbinger of changes to come from global warming, scientists on a mission to the frozen continent say.

Last February, a 2,500 sq km (965 sq m) iceberg broke off from a giant floating tongue of ice from the Mertz Glacier after being rammed by an even larger iceberg.

The ice tongue, sticking out into the Southern Ocean, had acted like a dam, preventing sea ice from moving into a permanently open section of water to the west.

But now with the ice tongue gone due the collision, scientists fear it could trigger changes to the behaviour of a major part of global ocean circulation patterns that shift heat around the globe via myriad currents at the surface and along the bottom.

The area around the glacier tongue, since halved in length by the collision, and to the west are one of the few places around Antarctica where dense, salty water is formed and sinks to the depths of the ocean, said mission leader Steve Rintoul on Monday.

The increasing number of icebergs breaking off Antarctica may have an unexpected benefit. According to one team of scientists, the bergs could feed carbon-loving plankton. If they are right, melting icebergs could – theoretically – slow global warming. Just how great an effect this would have remains to be seen.

Rob Raiswell of the University of Leeds, UK and colleagues trained high-resolution microscopes on ice sampled from icebergs in the Southern Ocean and the Antarctic glaciers from which they are born.

They found nano-sized particles of iron, between five and 10 millionths of a millimetre across. The team believe that because of the size and structure of the particles, the iron could be assimilated by phytoplankton.

“Most of the ground-up rock carried by icebergs is thought to be inert,” says Raiswell. “However, the high resolution microscopy shows there are small amounts of iron nanoparticles. They simply could not be seen except by these techniques.”

Phytoplankton need iron in order to grow, and the Southern Ocean is generally thought to be low on iron. But there is evidence that some Antarctic glaciers are flowing into the ocean faster because of climate change. This means more icebergs. If Raiswell’s findings are correct, more icebergs would mean more dissolved iron, therefore more phytoplankton, and more carbon dioxide sucked out of the atmosphere and into the oceans.

Plankton boost

“Dust has been thought to be the main outside source of iron to the Southern Ocean,” says Raiswell. He and his colleagues calculated that existing icebergs could double the supply of iron to the region.

Denman also points out that climatologists think there is typically less iron in the oceans during warm inter-glacial periods. “Why would human-induced warming increase the iron supply whereas recent natural warming occurred at the same time as decreased iron and southern Ocean [phytoplankton], as far as we can tell from the ice cores?”

It is too early to say how much of an impact more icebergs will have. One problem is that not all plankton sinks to the bottom of the ocean and contributes to the deep-ocean carbon sink. Part of it is eaten by marine animals and returned to the water column in their excrement. Geochemists have only a poor idea of the amount of carbon that is cycled in this way.